The present invention relates to a method and apparatus for producing a composite analog signal containing a plurality of tones wherein the phase of each tone represents data. More particularly, it relates to a multitone differentially encoded phase shift keyed (DPSK) modulator in which the frequency, amplitude and starting phase of each of the tones are programmable.
In phase shift keyed signal communication, a binary signal is transmitted by selectively changing the phase of a carrier signal by one of two predetermined phase shifts. In a receiver responsive to phase shift keyed signals, a demodulator is used to determine which of the two phase shifts occurred and thus which data bit (zero or one) was sent.
In differentially encoded phase shift keyed (DPSK) signal communication, data bits are taken one, two or more at a time and are represented in the transmission as one of a plurality of predetermined phase shifts in a carrier signal. The phase shifting is performed at predetermined intervals, often called the baud interval. For multitone DPSK communications, multiple carrier frequencies (or tones) are modulated by the binary data and the composite of these tones is transmitted. Multitone DPSK receivers recover the binary data by detecting the multiple carrier frequencies and then measuring the phase change, if any, in each of the carrier frequencies at the appropriate baud interval.
Digital multitone DPSK modulators are known in which each of the carrier frequencies is generated from a memory unit in which is stored a plurality of signals representing the amplitude of a single cycle of a sinusoidal function. A tone is synthesized by addressing successive signals in the sinusoidal memory unit at a given sampling rate and producing an analog signal equal to that specified by the memory unit. The frequency of the synthesized tone depends upon the sampling rate and the increments by which the sinusoidal memory unit is addressed. For a given sampling rate, the lowest (or "fundamental") frequency is synthesized by using single increments through the memory unit. Harmonics of the fundamental frequency are synthesized by incrementing through the memory in multiple increments. For example, the n.sup.th harmonic is generated by addressing every n.sup.th signal in the sinusoidal memory unit. Composite tones are simultaneously synthesized by summing the samples of each of the component tones during each sample period. This sum represents one output sample of the composite signal.
For digital multitone DPSK modulation, the synthesized tones are phased shifted once during each baud interval. The amount of such phase shift of each tone represents a predetermined number and value of input digital data bits. Generally, in order to provide for detection and discrimination at the receiver/demodulator, there are a plurality of sample intervals for each baud interval.
One problem with current digital multitone DPSK modulators is that, once built, the composition of the composite signal is not readily varied. Thus, in current DPSK modulators such as those disclosed in U.S. Pat. No. 4,270,208 to DeLellis the number of tones included in the composite analog signal, and the frequencies and starting phases of each of the tones are fixed. Such modulators can communicate only with receivers built to detect the identical number of tones, and the identical frequencies and starting phases of each of the tones.
Another similar problem with current digital multitone DPSK modulators is the difficulty encountered in changing the subset of harmonics of the fundamental frequency which is generated. Such limitations reduce the usefulness of conventional modulators because of their inability to communicate except with receivers made to respond to the identical tone libraries.
It is therefore an object of the present invention to obviate many of the problems associated with known systems and to provide a novel method and a digital input data responsive multitone DPSK signal generator.
It is another object of the present invention to provide a novel method and apparatus for generating a multitone DPSK signal in which the amplitude, frequency and starting phases of each of the tones can be independently programmed.
It is a further object of the present invention to provide a novel method and apparatus for generating a multitone DPSK signal in which the number of tones included within the composite signal can be independently programmed.
It is yet another object of the present invention to provide a novel method and apparatus for generating a multitone DPSK signal in which the frequencies of the component tones may be changed simply by the selection of an appropriate sample clock rate.
It is yet a further object of the present invention to provide a novel method and apparatus for synthesizing a composite multitone analog signal.
It is still another object of the present invention to provide a novel method and multitone DPSK modulator which is simple, flexible, and relatively inexpensive.
These and other objects and advantages of the present invention will become apparent to one skilled in the art from the claims and from a perusal of the following detailed description when read in conjunction with the attached drawings.